Typically, a vehicle is equipped with a DC-DC converter that boosts a direct current (DC) voltage of a vehicle battery to generate a driving voltage for driving a fuel injector. In a conventional DC-DC converter disclosed in, for example, U.S. Pat. No. 6,407,593 corresponding to JP-A-2001-73850, a switching element coupled to a vehicle battery via a coil is turned on and off so that counter-electromotive force can be produced in the coil. A capacitor is charged by the counter-electromotive force, thereby boosting a DC-DC voltage of the vehicle battery.
In the DC-DC converter, the driving voltage decreases after the fuel injector is driven. To prevent this problem, the DC-DC converter is configured such that as large an electric current as possible flows through the coil when the switching element is turned on. In such an approach, when the switching element is turned off, as large counter-electromotive force as possible is produced in the coil so that the driving voltage can rapidly reach a target voltage.
In the DC-DC converter, even when an engine of the vehicle stops, and the fuel injection value is not driven, the driving voltage decreases gradually due to self-discharge of the capacitor. To prevent this problem, the DC-DC converter is configured such that the above-described voltage boost operation is continued even during a period of time when the vehicle engine stops. In such an approach, the vehicle engine can be rapidly restarted.
As described above, since such a conventional DC-DC converter causes a large current to flow through a coil and a switching element, a considerable amount of heat is produced in the coil and the switching element. For example, the DC-DC converter is installed in an engine room of a vehicle. In such a case, while the vehicle is running, a large amount of air is introduced in the engine room. Therefore, the heat generated in the coil and the switching element is dissipated by the air flow so that the coil and switching element can be suitably cooled. However, when the vehicle stops, airflow enough to cool the coil and the switching element cannot be generated. As a result, a temperature of the DC-DC converter may increase significantly, and heat emitted from the DC-DC converter may affect electronic devices located near the DC-DC converter.